INHERITANCE IN ORGANISMS 539 



young usually produced are identical quadruplets resulting from the 

 development of a single zygote and possessing a common placenta. On 

 the other hand fraternal twins, being produced from two zygotes, may 

 differ in sex and in other characteristics to the same extent that two 

 offspring produced by the same parents at different times may differ. 



610. Determination of Sex in Parthenogenesis. — It appears from 

 what has just been stated that sex is determined at the time of fertiliza- 

 tion. This suggests the question of sex in parthenogenesis, but there is 

 little precise knowledge of the sex chromosomes in parthenogenetic types. 

 In all cases the parthenogenetic females have the diploid number of 

 chromosomes and when they produce another female generation no reduc- 

 tion division occurs and the diploid number is preserved. At certain 

 times males are produced and since reduction does occur in this case they 

 have the haploid number. They produce two kinds of sperm cells. The 

 development of those of one kind is arrested at some stage; they are 

 non-functional and die without uniting with an egg cell. Those of the 

 other kind do fertilize egg cells, which of course have the haploid number, 

 and from these zygotes females develop which have the diploid number of 

 chromosomes and which may initiate other lines of parthenogenetic 

 female generations. It is known that in the case of the honeybee a 

 fertilized egg produces a female, with the diploid number of chromosomes, 

 and an unfertilized egg a male, with the haploid number. It has been 

 suggested that the male may have one sex chromosome and that all the 

 functional sperm cells possess one, those with none not developing. The 

 female would have two sex chromosomes and all unfertilized egg cells 

 would have one. This would explain the uniform development of females 

 from fertilized egg cells. In the rotifers two kinds of eggs are produced, 

 those of one kind undergoing no reduction and therefore developing 

 females, while those of the other kind undergo reduction and develop 

 into females or males, depending on whether or not they are fertilized. 



In the case of parthenogenetic types it has been shown that feeding 

 has an effect upon sex. Whitney has found that when rotifers are fed 

 an abundance of green flagellates the number of males in the F2 generation 

 is increased, while if colorless flagellates are fed, the reverse is the case. 

 The effect may be exerted through modifying the tendency to chromosome 

 reduction. 



611. Sex-linked Characters. — The sex chromosomes not only deter- 

 mine the sex but also carry genes for many other characters. It thus 

 follows that these characters will be determined when sex is determined 

 and will be associated with one sex or the other. In the case of color 

 blindness in man the defect is carried in the .r-chromosome in the male 

 and is recessive, while normal vision, which is dominant, is carried in the 

 a--chromosor»e in the female. The sex chromosome carrying the defect 

 may be indicated by x. A color-blind father (xy) and a normal mother 



